1. Field of the Invention
The present invention relates to a surface light source device of a type in which a primary light source is disposed at the side of a light guide plate and one surface of the light guide plate serves as a luminous surface, i.e., a "surface light source device of side light type", and more particularly to a side-light-type surface light source device for improving a distribution pattern of a light diffusing element on a rear surface of the light guide plate. The side-light-type surface light source device according to the present invention is particularly useful when applied to back lighting of a liquid crystal display.
2. Related Art
A general structure of a surface light source device of a side light type and its operation will be described. FIG. 1 is a fragmentary cross-sectional view showing the structure of a known surface light source device of side light type. In FIG. 1, a reference numeral 2 designates a light guide plate usually made of transparent plastic material. At the side of the light guide plate 2, a rod-shaped light source element 1 is disposed along a side edge surface (an incidence surface) 2a of the light guide plate 2, as a primary light source. Generally, a cold-cathode tube is used as the light source element 1. The light source element 1 is hereinafter simply referred to as a lamp.
One surface of the light guide plate 2 serves as a luminous surface on which at least one sheet-shaped element 3 (hereinafter referred to as "an additional element") is additionally disposed. The additional element 3 is provided in the form of a light diffusing sheet, a prism sheet, a prism sheet protection sheet or an interference fringe prevention sheet, etc.
On a surface opposite to the luminous surface (hereafter referred to as "a rear surface" and also referred to as a reflecting surface), a reflecting sheet 4 is disposed. The reflecting sheet 4 is provided in the form of a silver foil, an aluminum foil or a white sheet. At the back surface of the lamp 1, a non-illustrated reflector is disposed for improving the efficiency of incidence of light on the light guide plate 2.
A major part of the light emitted from the lamp 1 is introduced into the light guide plate 2 directly or indirectly from the incidence surface 2a. The light introduced into the light guide plate 2 is guided toward the back surface opposite to the incidence surface 2a as repeatedly reflected on the front and rear surfaces of the light guide plate 2. The light is gradually taken out from the luminance surface at this stage and emitted as illuminating light as being subjected to an action (a light diffusing action) of the additional element 3. This illuminating light is utilized as, for example, a back lighting of a liquid crystal display.
The foregoing are the most basic structure and operation of the surface light source device of a side light type. With the simple structure and operation as described above, it is difficult to secure evenness of luminance of the upper surface of the additional element 3, which is served as a luminous surface. Specifically, a gradient of the luminance is caused on the luminous surface according the distance from the incidence surface 2a. Further, the function of directing the light introduced into the light guide plate 2 to the luminous surface is inadequate, so that the luminance level tends to be deficient. In order to avoid this problem, it has been adopted to arrange a number of light-diffusive elements in a predetermined distribution pattern on the rear surface of the light guide plate 2. As a number of dot-shaped elements are arranged according to a predetermined concept, the distribution pattern is called a dot pattern.
FIG. 2 is a plan view, as seen from the rear side, of the light guide plate 2 on which the light diffusing elements are arranged in a most basic distribution pattern. In FIG. 2, large and small circles show light diffusing elements in the form of a print layer of light-diffusive ink, an aventurine surface (a finely roughed surface), etc. A number of light diffusing elements are arranged in rows parallel to the lamp 1. A density of existence of light diffusing elements (a rate of area occupation of light-diffusive region) decreases gradually according to the distance from the lamp 1.
Adopting the light guide plate 2 as shown in FIG. 2, it is possible to suppress the luminance difference depending on the distance from the lamp 1. However, the luminance of a portion (indicated by S) near the electrode portion 1a of both ends of the lamp 1 would be insufficient. For example, in the case of a lamp of 217 mm in full length, which is available on the market, the length of an emitting portion is 203 mm and there is an electrode portion (a non-emitting portion) of 7 mm wide at each end of the lamp. In the vicinity of such a non-emitting portion, the luminance would be insufficient as a matter of course.
If the length of the emitting portion of the lamp 1 is set substantially equal to the width d of the light guide plate 2, it is possible to somehow minimize the foregoing problem and, on the other hand, adopting a lamp having the emitting portion of such length is very unpreferable in view of making both shape and size of the surface light source device compact.
Various proposals have been made to overcome the above problems. FIG. 3 illustrates one of the proposals. In this conventional art, two corner portions F of the rear surface of the light guide plate 2 shown in FIG. 2, which are nearer to the electrode portion 1a of the lamp 1 are treated differently from the other corner portions with respect to width t1 and depth t2. Namely, in these corner portions F, the light diffusing elements are arranged in an increased density as compared with the circumference thereof. Within the corner portions F, as shown, density of the light diffusing elements may also be graded in the depthwise direction, if necessary.
However, this method of defining the distribution pattern also involves a problem. Namely, since the corner portions F are treated specially, it is inevitable that a sharp difference occurs in density of occupation of light diffusing elements between portions contiguous to these corners F. This difference may cause unevenness of luminance in the vicinity of the corner portions.
FIG. 4 illustrates another conventional art according to a different concept from the example shown in FIG. 3. In this conventional art, the rear surface of the light guide plate 2 shown. in FIG. 2 is divided into a region I near the lamp 1 and a region II far from the lamp 1. In the region I, the light diffusing elements are arranged in such a manner that they radially rodite about a point G which confronts a central point of the lamp 1. Namely, within the region I, the distribution pattern is defined so that the density of occupation of light diffusing elements is minimal in the vicinity of the point G, and gradually increases in accordance with the distance from the point G.
Meanwhile, within the region II, a concept similar to that in FIG. 2 is adopted. Namely, the light diffusing elements are arranged in rows parallel to the lamp 1, and its density of occupation decreases gradually according to the distance from the lamp 1.
This method of defining the distribution pattern has another problem. Namely, since the density of occupation of light diffusing elements is graded radially in the region I while it is graded linearly in the depthwise direction of the light guide plate 2 in the region II, the two patterns can not be matched with each other in the vicinity of the junction of the regions I and II, so that a smooth variation of density of occupation of the light diffusing elements is not achieved. This mismatching might cause the luminance unevenness in the vicinity of the junction of the regions I and II.
Further, if the gradient in the region I is set so as not to cause inadequate luminance in the vicinity of the electrode portion 1a and the magnitude and gradient of the density of occupation of the light diffusing elements are set independently with its appropriateness taken precedence of the other things in the region II, portions indicated by Z might be reversely graded in density of occupation of the light diffusing elements.
Thus, in the conventional art, if the length of the emitting portion of the rod-shaped lamp disposed at the side of the light guide plate is smaller than the width of the light guide plate, it is difficult to define an appropriate distribution pattern of light diffusing elements on the rear surface of the light guide plate, which secures the uniform luminance in both depthwise and widthwise directions of the light guide plate and prevents local occurrence of sharp gradients and reverse gradients of the density of occupation of the light diffusing elements.